[PATCH 1/3] sched: Introduce a new flag PF_DONT_HUNG.

[Julian Sun <sunjunchao@bytedance.com>]

In the kernel, long waits can trigger hung task warnings. However, some
warnings are undesirable and unnecessary - for example, a hung task
warning triggered when a background kworker waits for writeback
completion during resource cleanup(like the context of
mem_cgroup_css_free()). This kworker does not affect any user behavior
and there is no erroneous behavior at the kernel code level, yet it
triggers an annoying hung task warning.

This patch adds a new flag to task_struct to instruct the hung task
detector to ignore such cases, as suggested by Andrew Morton in [1].
Also introduces current_set/clear_flags() to prepare for the next patch.

[1]: https://lore.kernel.org/cgroups/20250917152155.5a8ddb3e4ff813289ea0b4c9@linux-foundation.org/

Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
---
 include/linux/sched.h | 12 +++++++++++-
 kernel/hung_task.c    |  6 ++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index f8188b833350..cd70ff051b2a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1718,6 +1718,16 @@ static inline char task_state_to_char(struct task_struct *tsk)
 	return task_index_to_char(task_state_index(tsk));
 }
 
+static inline void current_set_flags(unsigned int flags)
+{
+	current->flags |= flags;
+}
+
+static inline void current_clear_flags(unsigned int flags)
+{
+	current->flags &= ~flags;
+}
+
 extern struct pid *cad_pid;
 
 /*
@@ -1747,7 +1757,7 @@ extern struct pid *cad_pid;
 						 * I am cleaning dirty pages from some other bdi. */
 #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
 #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
-#define PF__HOLE__00800000	0x00800000
+#define PF_DONT_HUNG		0x00800000	/* Don't trigger hung task warning when waiting for something */
 #define PF__HOLE__01000000	0x01000000
 #define PF__HOLE__02000000	0x02000000
 #define PF_NO_SETAFFINITY	0x04000000	/* Userland is not allowed to meddle with cpus_mask */
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 8708a1205f82..b16d72276d01 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -208,6 +208,12 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 		t->last_switch_time = jiffies;
 		return;
 	}
+
+	if (t->flags & PF_DONT_HUNG) {
+		t->last_switch_time = jiffies;
+		return;
+	}
+
 	if (time_is_after_jiffies(t->last_switch_time + timeout * HZ))
 		return;
 
-- 
2.39.5